1. Field of the Invention
This invention pertains to methods and apparatus for wrapping a film of wrapping material around an object at a wrapping station and, more particularly, to improved apparatus for controlling the relative position of a cutting and sealing bar on the film during the cutting and sealing operation. In addition, the invention pertains to a method and apparatus for controlling the feed of lightweight films of wrapping material which are sealed together in the form of a web and drawn about an object to be wrapped.
2. Description of the Prior Art
Various types of devices have been used heretofore to wrap an object with a heat-sealable film or wrapping material. Generally, these devices supply wrapping material from two vertically spaced rolls sealing them together at a wrapping station to form a web. An object is then conveyed through the wrapping station where it engages the web pulling additional wrapping material from the supply rolls and a heat-sealing and cutting unit travels down behind the object to seal and cut the wrap encircling the object. The sealing and cutting unit separates the wrap on the object from the wrap on the supply rolls and simultaneously forms a new web at the wrapping station. Typical of these devices is the one shown in U.S. Pat. No. 3,551,260 granted to Catherwood. One difficulty in providing a good seal on the wrap around the object or on the new web is to avoid relative movement during the sealing and cutting operation between the sealing and cutting unit and a resilient roll against which the unit presses the wrap during cutting and sealing. As can be seen in the Catherwood patent, attempts to reach the same relative velocities have been met with complicated mechanism which, while attempting to generally reach identical speeds, have not, in fact, achieved such identical velocities and as a result, have not achieved satisfactory sealing of the wrap.
Still another problem with the sealing and cutting apparatus of the prior art has been the difficulty to accommodate irregularities in the periphery of the resilient sealing roll to obtain uniformity of sealing and cutting pressure transversely across the entire length of the wrap. In the prior art, irregularities in the peripheral surface of the roll has resulted in variations in the pressure between the cutting and sealing unit and the resilient roll resulting in inadequate sealing pressure applied at some parts of the wrap and excessive pressure at others, thus resulting frequently in an unsatisfactory seal.
Still another problem in prior art sealing and cutting apparatus is that production rates and sizes of objects being handled in prior art apparatus have been limited. For example, in the Catherwood U.S. Pat. No. 3,551,260, the sealing and cutter unit follows an elliptical path to increase the height of the objects being handled but this elliptical path uses too great a time to complete to reach high desirable production rates such as 60 objects per minute.
Still an additional problem in prior art wrapping methods and apparatuses is caused by the difficulty in handling polyethylene or other lightweight heat-sealable wrap or films and to allow them to move freely out of the supply while the object is being wrapped.